This invention relates to new corrosion-inhibiting aqueous solutions, anticorrosives and antifreeze agents, and to their production.
Water is used in a steadily increasing number of heat-transfer systems and heat exchangers, for example motor-vehicle radiators and hot-water heating systems. However, disadvantages of using water in such systems include its freezing point which in the winter months is frequently above the outside temperature, and its corrosive effect upon the metal surfaces of the heat-transfer systems.
Antifreeze agents based on glycols which are commercially available have been effectively used to lower the freezing point of water.
It is known that iron alloys for example, cast iron and mild steel, quickly corrode or rust when in contact with water and in the presence of air. It is also known that the rusting or corrosion of iron and other metals in water is promoted by glycols. In order to prevent this corrosion, a number of chemicals have already been proposed for use as corrosion inhibitors, including, for example, soda, sodium nitrite, sodium nitrate, borax, sodium dichromate, alkali salts of arsenous acid, arsenic acid, phosphorous acid, phosphoric acid, alkali tungstates, and also alkali salts of weak acids, especially benzoic acid and butyl benzoic acid, and phenols, aliphatic, alicyclic and heterocyclic amines, alkanolamines, especially triethanolamine, and organosilicon compounds. Unfortunately, none of these compounds is effective on its own, so that it has been proposed to use mixtures of several components in definite ratios for practical application (German Patents Nos. 1,154,976 and 1,176,930, DAS No. 1,201,121).
The question of choice is met with difficulties from two different directions. On the one hand, it is not only one metal, but several different metals or alloys differing in their corrosion behaviour which have to be protected in a heat-transfer or cooling system, especially in motor vehicles. On the other hand, the individual corrosion inhibitors intervene differently in the process of corrosion with the result that they are not uniform in their protective effect on the various metals or alloys. Accordingly, it is necessary to select a mixture of individual corrosion inhibitors (DAS No. 1,201,121, Col. 1 lines 3 - 23).
It is also known that it is not possible to eliminate the unsatisfactory properties of a mixture simply by adding other inhibitors, and that, in practice, the favourable properties already obtained are often lost when other components are added (German Patent No. 1,154,976 Col. 1, line 49 to Col. 2, line 23).
Over recent years, the increasing use of aluminium alloys and the use of sealed cooling systems in motor vehicles have imposed more stringent requirements upon anticorrosives in the form of protection against corrosion for lightweight metal alloys and long-term activity. The addition of a nitrophenol, preferably of picric acid, has recently been proposed to meet the second of these two requirements (British Patent No. 1,221,996). None of the known agents affords the necessary protection against corrosion for lightweight metal alloys so that there is an obvious gap and a genuine need in their case (see Col. 1 lines 20-45 of the British patent).
It is also known that one particular difficulty in protecting metal castings against corrosion is that the chemical composition of the casting is not uniform throughout so that a conventional anticorrosive mixture has a totally different and, in many cases, inadequate effect on different parts of the casting. When a mixture of this kind is tested, the level of corrosion detected differs according to the place at which the sample was taken from the casting, and despite a basically satisfactory result of the mixture outliers are frequently detected in the negative sense.
Accordingly, there is considerable scope and an urgent need for improvements in the field of corrosion prevention in heat-transfer systems which use water as the heat-transfer medium.